1. Field of the Invention
This invention relates to the recovery of oil from subterranean reservoirs of varying permeability.
2. Description of the Prior Art
Since only a portion of the oil contained in a petroleum reservoir can be recovered by primary methods, it has become conventional practice to employ various secondary and tertiary recovery techniques to produce additional quantities of oil not economically recoverable by primary means. One of the various secondary and tertiary recovery methods available is to displace the oil through the reservoir with a driving fluid such as water or steam injected for that purpose. Normally in carrying out the flooding processes, a series of input wells placed apart from one or more producing wells are drilled into and opened to the oil producing strata. The injection well locations with reference to the production well are selected to afford a desired flood pattern. This selected pattern, depends in part upon field conditions, the location of existing wells, and the operator's preference. Aqueous drive fluid such as steam, water, brine, or a viscous, aqueous polymer solution is forced into the input wells under pressure and through the surrounding oil bearing strata toward the producing well or wells. While water flooding has been rather widely practiced in recent years, it is not without considerable operating problems and economic limitations, particularly those associated with low oil recovery in proportion to the amount of water injected. Various surfactant and solvent fluids have been proposed as means for recovering additional quantities of oil over that recoverable by conventional water flooding. However these processes face serious operating problems when practiced in heterogeneous formations containing strata or channels having permeability substantially higher than the bulk of the formation. Similar problems occur with steam flooding.
One of the major problems encountered in a flooding operation is breakthrough of the flooding medium from the flood front to the producing well relatively early in the displacement process and rapidly increasing water/oil ratio at the production well following the initial breakthrough. These difficulties result from displacing the medium channeling through a portion of the oil bearing structure to the producing well thus bypassing large zones of the oil bearing strata. The reason for the channeling of the flooding medium to the producing well and the resulting low oil recovery is due in part to the peculiar structure of the oil bearing strata. Underground oil reservoirs in most cases consist of layers of sand or rock and since no reservoir rock is perfectly uniform in composition and structure, permeability will vary across the rock face or strata. Also, fractures, cracks, vugs and other anomallies can promote channeling of the displacement fluid.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid fills up in a wide bank in front of the driving fluid which moves uniformly toward the producing well. To keep this bank of oil intact and constantly moving toward the producing well, a substantially uniform permeability must exist throughout the strata. If this uniform permeability does not exist or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with a consequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing wells. Moreover, as the more permeable strata are depleted, the driving fluid has a tendency to follow channels and further increase consumption of the flooding medium to the point where the process becomes uneconomical. The maximum limit in terms of recovered oil can be as high as 100 barrels of driving fluid per barrel of oil. It is, of course, desirable to operate at much lower driving fluid to oil ratios and preferably not more than 15 and normally a maximum of 5 to 10 barrels of driving fluid per barrel of recovered oil is considered an acceptable operating condition, particularly where the driving fluid is a low cost agent such as flood water. While a uniform flood front with reduced fingering can be obtained in some formations with a drive fluid rendered more viscous by the addition of various water soluble polymers or other material capable of imparting higher viscosity to the flood water, viscous water flooding is often ineffective in formations having severe heterogeniety as the mobility of the flood water cannot be economically reduced sufficiently to prevent or substantially restrict channeling of the flooding medium. Also, smaller volumes of aqueous polymer solution are sometimes injected into a heterogeneous reservoir to divert subsequently injected flooding media into the less permeable strata, however, such treatments are often ineffective or only partially effective in highly stratified formations. Where a heterogeneous formation is flooded, it is usually found that the flood water or other flooding medium which is introduced into the injection well will enter the various strata open to the well at different flow rates depending upon the permeability of the individual strata relative to the permeabilities of other strata in the formation. The water injection well of this type is said to exhibit a non-uniform injection profile. Heretofore it has been considered that a non-uniform injection profile was symptomatic of channeling and the resulting poor flood medium conformance. Accordingly, it was believed that selective plugging or other treatment to improve or correct the injection profile would ameliorate channeling and result in improved flood water conformance and increase oil recovery. However, most commercially practical selective plugging treatments effect the permeability of the strata only at the well face or for a few feet distance from the well. Since adjacent strata are often in fluid communication throughout all the rest of the substantial part of the formation subjected to flooding, crossflow between the various strata exists and severe channeling develops even though a substantially uniform injection profile is obtained. Thus, in many applications, treatment of the injection wells to improve the water injection profile has no demonstrable effect on oil recovery. Hence, need exists for an improved flooding process for use in heterogeneous formations that will minimize channeling of the flooding medium or bypassing at large areas of the formation, or for a method for improving the effectiveness of the conventional selective plugging process. Accordingly, a principal object of this invention is to provide an improved flooding process for recovering oil from hetergenous petroleum reservoirs. Another object of this invention is to provide an improved method whereby water flooding processes for recovering oil from reservoirs having strata of widely varying permeability may sweep the maximum amount of reservoir possible and not channel into the highly permeable zones.